He may have finished last in Iran's disputed presidential election, but in the weeks that followed, Mehdi Karroubi has often taken the lead in challenging the Iranian government. After the announcement of the result triggered massive demonstrations in June, Karroubi was one of the first major figures to blame the government for the violence — a brave act considering that the state media was calling the demonstrations riots instigated by foreign powers. And when Basiji militiamen roughed him up on the way to Friday prayers last month, Karroubi spoke out again. "They want to create an atmosphere of threat and terror so that people are kept silent," he said. And despite the growing atmosphere of official intolerance for challenges to the postelection order, Karroubi has again infuriated supporters of President Mahmoud Ahmadinejad by publicizing the charge that opposition protesters were raped and abused in prison.While more powerful reformists than Karroubi have stuck to vague appeals for justice, Karroubi is confronting the government with specific charges and has advanced a clear political platform, making himself a lightning rod for criticism from within the regime. And it's far from clear that Karroubi's more confrontational approach will yield results against a government willing to deploy overwhelming force to maintain its grip on power. Over the weekend, in fact, opposition activists were dismayed to hear reports that former President Ali Akbar Hashemi Rafsanjani had called for unity and for all Iranians to follow Supreme Leader Ayatullah Ali Khamenei's orders to end the turmoil. Coming a little over a month after the influential cleric and Ahmadinejad rival had openly questioned June's disputed election results, the comments suggested that the opposition movement could be losing one of its most crucial establishment backers.Still, the newfound role of renegade seems to suit Karroubi, the 72-year-old cleric who once enjoyed impeccable credentials within the Islamic Republic. Indeed, Karroubi is a battle-hardened revolutionary, having served time in prison during the Shah's reign, studied under Ayatullah Khomeini and served as speaker of parliament until 2004. In 2005, he placed third in the first round of presidential elections, losing out to then mayor of Tehran Mahmoud Ahmadinejad, in a race that Karroubi later alleged was rigged. After his defeat in 2005, he remained on the offensive, forming a political party and newspaper to challenge Ahmadinejad. Now that Karroubi finds himself on the bitter end of yet another contested election, he's using that political base to chart a course separate from that of the rest of Iran's reformist opposition.That Karroubi is a different kind of reformist became clear during this year's presidential campaign. While Mir-Hossein Mousavi became the opposition front-runner in large part because he was the best-known reformist in the race, his popularity in Iran stems mostly from the fact that he is not Mahmoud Ahmadinejad. On the other hand, Karroubi, though less well-known, attracted a circle of advisers from among the country's most respected reformist technocrats, and ran on a specific program of reforms targeted at specific electoral groups such as women, students and the non-Persian minorities who make up close to half of Iran's population. Along with policies supporting fiscal restraint and strengthening the rule of law, Karroubi promised that, if elected, he would sign Iran up to international protocols on women's rights, and would end patrols by the country's religious police, who enforce Islamic dress codes for women.Now, as the reformists struggle to keep the opposition momentum alive at a time when government forces are making it nearly impossible to demonstrate on the streets, Karroubi is taking a different tack from Mousavi. The opposition front-runner announced the creation of a new movement, the Path of Green Hope, but as a broad social movement rather than a full-fledged political party. Karroubi, for his part, has said that he won't be joining the Path of Green Hope. Instead, he's focusing his efforts on holding the government on an issue that has clearly resonated with the Iranian public — the allegations of rape and abuse of opposition supporters arrested in the postelection crackdown.The controversy began in July when Karroubi wrote a letter to Rafsanjani detailing allegations from witnesses and victims. "Some of the detainees claim incarcerated girls were raped so harshly until their uteruses were torn apart, while young boys were sexually abused so savagely that they are suffering from serious depression as well as physical and mental traumas," Karroubi wrote. "Those who have been subject to these embarrassing tortures have been threatened with death if they disclose details." Since then, Karroubi has collected evidence of the abuse, and on Aug. 19 he called for a meeting of the three branches of government to review the charges.Karroubi is convinced that despite its monopoly over the security services, the Ahmadinejad government will eventually be unable to withstand the tide of ill will generated by accusations of election-rigging, prisoner abuse and forced confessions. "This group succeeded in grabbing power, but can they solve the problems? Satisfy the people? Have good relations with the world? Solve unemployment?" he told the Washington Post Aug. 18. "Problems will not be solved, and people will be more unhappy."Perhaps, but if the government feels truly threatened, it's likely to lash out again — and Karroubi could be among its top targets. One of the leading generals of Iran's powerful Revolutionary Guards has already called for his and Mousavi's arrest. Karroubi's newspaper was shut down on Aug. 18, and last week the government advised clerics all over the country to lambaste Karroubi during their Friday sermons. One leading hard-line cleric called for Karroubi to be beaten with a lash.But having risked his life and freedom in the struggle against the Shah, Karroubi appears unbowed by the threats. And he may be calculating that the regime will do itself even more damage if it arrests a man blowing the whistle on the treatment of opposition supporters behind bars in Iran.